This invention relates to a linear drive, and more particularly, to a linear drive that prevents jamming at a hard endstop upon being reversed.
In many applications, a movable structure is moved linearly until it reaches a limiting stationary structure. In an exemplary application of particular interest to the inventor, a frame holding a lens is moved linearly, within limits, in a projector as the projector is tilted, to minimize distortion of the image projected through the lens. The linear movement must be reversible, to account for a tilt back to the original position.
If the movable structure is driven continuously into the stationary structure, one or both may be damaged, or the movable structure may be jammed against the stationary structure so that it cannot be reversed. To prevent these problems, some type of limiting mechanism is usually provided. In one approach, the drive motor is sized to be sufficiently small that it cannot drive the movable structure into the stationary limiting structure with sufficient force to damage them. This technique may result in jamming of the movable structure against the stationary structure, with the drive motor having insufficient reversing force from the stalled condition to back the movable structure away from the stationary structure.
In another approach, range limiting microswitches are positioned so that the drive motor is stopped at a preselected location before the moving element is jammed into the stationary limiting structure. Failure or misadjustment, either initially or over time, of the microswitch causes a failure which may render the device inoperable or result in damage the structure.
In yet another approach, a torque-limiting clutch is placed into the drive train of the drive motor so that, when the movable structure encounters the stationary structure, the maximum force that can be exerted between the two is limited. The use of the limiting clutch has the drawback that the electronic drive must "spike" the power to the motor power supply to obtain a higher torque to unstick the leadscrew from the jammed condition, when the motor is reversed. Further, the clutch must remain properly adjusted, or the limiting effect is lost. Normal wear over time may be sufficient to lose the beneficial effect of the clutch.
In these approaches, and others as well, the limiting mechanism adds weight and cost to the structure and product, and is a potential source of failure during service. There is therefore a need for an improved approach to linear drives whose extent of travel must be limited, and which must be reversing. The present invention fulfills this need, and further provides related advantages.